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37-666: It is an accretionary wedge caused by the African Plate subducting under the Eurasian and Anatolian plates. As the African Plate moves slowly north-northeastward, the sedimentary rocks covering the Mediterranean seafloor are being affected by active shortening, involving both thrust faulting and folding , lifting them up and forming the ridge. Along the ridge, five deep basins full of anoxic brine have been found (including

74-480: A ramp and typically forms at an angle of about 15°–30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as a ramp anticline or, more generally, as a fault-bend fold . Fault-propagation folds form at the tip of a thrust fault where propagation along the decollement has ceased, but displacement on the thrust behind the fault tip continues. The formation of an asymmetric anticline-syncline fold pair accommodates

111-500: A back-arc or a forearc basin. It was later accreted to the continental margin of Laurasia. Longitudinal sedimentary tapering of pre-orogenic sediments correlates strongly with curvature of the submarine frontal accretionary belt in the South China Sea margin, suggesting that pre-orogenic sediment thickness is the major control on the geometry of frontal structures. The preexisting South China Sea slope that lies obliquely in front of

148-497: A decrease in stable taper angle from 8.4°–12.5° to <2.5–5°. In general, low-permeability and thick incoming sediment sustain high pore pressures consistent with shallowly tapered geometry, whereas high-permeability and thin incoming sediment should result in steep geometry. Active margins characterized by a significant proportion of fine-grained sediment within the incoming section, such as northern Antilles and eastern Nankai , exhibit thin taper angles, whereas those characterized by

185-642: A dynamically maintained response to factors which drive pore pressure (source terms) and those that limit flow (permeability and drainage path length). Sediment permeability and incoming sediment thickness are the most important factors, whereas fault permeability and the partitioning of sediment have a small effect. In one such study, it was found that as sediment permeability is increased, pore pressure decreases from near-lithostatic to hydrostatic values and allows stable taper angles to increase from ~2.5° to 8°–12.5°. With increased sediment thickness (from 100–8,000 m (330–26,250 ft)), increased pore pressure drives

222-402: A higher proportion of sandy turbidites, such as Cascadia , Chile , and Mexico , have steep taper angles. Observations from active margins also indicate a strong trend of decreasing taper angle (from >15° to <4°) with increased sediment thickness (from <1 to 7 km). Rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it

259-427: A mature wedge that has an equilibrium triangular cross-sectional shape of a critical taper . Once the wedge reaches a critical taper, it will maintain that geometry and grow only into a larger similar triangle . The small sections of oceanic crust that are thrust over the overriding plate are said to be obducted . Where this occurs, rare slices of ocean crust, known as ophiolites , are preserved on land. They provide

296-402: A sedimentary sequence, such as mudstones or halite layers; these parts of the thrust are called decollements . If the effectiveness of the decollement becomes reduced, the thrust will tend to cut up the section to a higher stratigraphic level until it reaches another effective decollement where it can continue as bedding parallel flat. The part of the thrust linking the two flats is known as

333-408: A sedimentary sequence, such as the top and base of a relatively strong sandstone layer bounded by two relatively weak mudstone layers. When a thrust that has propagated along the lower detachment, known as the floor thrust , cuts up to the upper detachment, known as the roof thrust , it forms a ramp within the stronger layer. With continued displacement on the thrust, higher stresses are developed in

370-467: A specific oceanic location or ocean current is a stub . You can help Misplaced Pages by expanding it . This tectonics article is a stub . You can help Misplaced Pages by expanding it . Accretionary wedge An accretionary wedge or accretionary prism forms from sediments accreted onto the non- subducting tectonic plate at a convergent plate boundary . Most of the material in the accretionary wedge consists of marine sediments scraped off from

407-544: A valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land. A classic example is the Coast Range ophiolite of California, which is one of the most extensive ophiolite terranes in North America. This oceanic crust likely formed during the middle Jurassic Period, roughly 170 million years ago, in an extensional regime within either

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444-399: Is a type of reverse fault that has a dip of 45 degrees or less. If the angle of the fault plane is lower (often less than 15 degrees from the horizontal ) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust or overthrust fault . Erosion can remove part of the overlying block, creating a fenster (or window ) – when

481-431: Is similar to that found in a thin-skinned foreland thrust belt. A series of thrusts verging towards the trench are formed with the youngest most outboard structures progressively uplifting the older more inboard thrusts. The shape of the wedge is determined by how readily the wedge will fail along its basal decollement and in its interior; this is highly sensitive to pore fluid pressure . This failure will result in

518-414: Is sufficient to cause dilatant fracturing. Dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid. Dilatant fracturing will create escape routes, so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile (dilatant) fracture, which is slightly in excess of

555-416: Is typically a lozenge-shaped duplex. Most duplexes have only small displacements on the bounding faults between the horses, which dip away from the foreland. Occasionally, the displacement on the individual horses is more significant, such that each horse lies more or less vertically above the other; this is known as an antiformal stack or imbricate stack . If the individual displacements are still greater,

592-601: The Alps , and the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults. Thrust faults occur in the foreland basin , marginal to orogenic belts. Here, compression does not result in appreciable mountain building, which is mostly accommodated by folding and stacking of thrusts. Instead, thrust faults generally cause a thickening of the stratigraphic section . When thrusts are developed in orogens formed in previously rifted margins, inversion of

629-475: The Cyrenaica peninsula. Detailed bathymetric mapping using multibeam echosounders , shows that deformation within the "outer zone" (southernmost part) of the ridge, is much more intense against the promontory, with out-of-sequence thrusting replacing the gentle folding observed further east and west. 33°30′N 25°00′E  /  33.5°N 25.0°E  / 33.5; 25.0 This article about

666-533: The L'Atalante basin ), where Messinian evaporite deposits of brine caught up in this ongoing orogeny have dissolved. In the far future, it could form part of a long high mountain range as the continued northward movement of the African Plate obliterates the eastern part of the Mediterranean Sea. The central section of the Mediterranean Ridge shows evidence for the initial stages of collision with

703-505: The geological basement of Japan is made up of accretionary complexes. Accretionary wedges and accreted terranes are not equivalent to tectonic plates, but rather are associated with tectonic plates and accrete as a result of tectonic collision. Materials incorporated in accretionary wedges include: Elevated regions within the ocean basins such as linear island chains, ocean ridges , and small crustal fragments (such as Madagascar or Japan), known as terranes , are transported toward

740-412: The ocean trench margin of subduction zones, where oceanic sediments are scraped off the subducted plate and accumulate. Here, the accretionary wedge must thicken by up to 200%, and this is achieved by stacking thrust fault upon thrust fault in a melange of disrupted rock, often with chaotic folding. Here, ramp flat geometries are not usually observed because the compressional force is at a steep angle to

777-543: The advancing accretionary wedge has impeded the advancing of frontal folds resulting in a successive termination of folds against and along strike of the South China Sea slope. The existence of the South China Sea slope also leads the strike of impinging folds with NNW-trend to turn more sharply to a NE-strike, parallel to strike of the South China Sea slope. Analysis shows that the pre-orogenic mechanical/crustal heterogeneities and seafloor morphology exert strong controls on

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814-514: The buried paleo-rifts can induce the nucleation of thrust ramps. Foreland basin thrusts also usually observe the ramp-flat geometry, with thrusts propagating within units at very low angle "flats" (at 1–5 degrees) and then moving up-section in steeper ramps (at 5–20 degrees) where they offset stratigraphic units. Thrusts have also been detected in cratonic settings, where "far-foreland" deformation has advanced into intracontinental areas. Thrusts and duplexes are also found in accretionary wedges in

851-438: The cohesive strength of the weak sediment layer that acts as the basal detachment. These assumptions allow the application of a simple plastic continuum model, which successfully predicts the observed gently convex taper of accretionary wedges. Pelayo and Weins have postulated that some tsunami events have resulted from rupture through the sedimentary rock along the basal decollement of an accretionary wedge. Backthrusting of

888-446: The continuing displacement. As displacement continues, the thrust tip starts to propagate along the axis of the syncline. Such structures are also known as tip-line folds . Eventually, the propagating thrust tip may reach another effective decollement layer, and a composite fold structure will develop with fault-bending and fault-propagation folds' characteristics. Duplexes occur where two decollement levels are close to each other within

925-440: The downgoing slab of oceanic crust , but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate. An accretionary complex is a current (in modern use) or former accretionary wedge. Accretionary complexes are typically made up of a mix of turbidites of terrestrial material, basalts from the ocean floor , and pelagic and hemipelagic sediments . For example, most of

962-429: The footwall of the ramp due to the bend on the fault. This may cause renewed propagation along the floor thrust until it again cuts up to join the roof thrust. Further displacement then takes place via the newly created ramp. This process may repeat many times, forming a series of fault-bounded thrust slices known as imbricates or horses , each with the geometry of a fault-bend fold of small displacement. The final result

999-505: The horses have a foreland dip. Duplexing is a very efficient mechanism of accommodating the shortening of the crust by thickening the section rather than by folding and deformation. Large overthrust faults occur in areas that have undergone great compressional forces. These conditions exist in the orogenic belts that result from either two continental tectonic collisions or from subduction zone accretion. The resultant compressional forces produce mountain ranges. The Himalayas ,

1036-421: The load pressure if the maximum compression is nearly horizontal. This in turn buffers the strength of the wedge at the cohesive strength, which is not pressure-dependent, and will not vary greatly throughout the wedge. Near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge. The shear resistance on the base of the wedge will also be fairly constant and related to

1073-513: The rear of the accretionary wedge, arcward over the rocks of the forearc basin, is a common aspect of accretionary tectonics. An older assumption that backstops of accretionary wedges dip back toward the arc, and that accreted material is emplaced below such backstops, is contradicted by observations from many active forearcs that indicate (1) backthrusting is common, (2) forearc basins are nearly ubiquitous associates of accretionary wedges, and (3) forearc basement, where imaged, appears to diverge from

1110-847: The sedimentary layering. Thrust faults were unrecognised until the work of Arnold Escher von der Linth , Albert Heim and Marcel Alexandre Bertrand in the Alps working on the Glarus Thrust ; Charles Lapworth , Ben Peach and John Horne working on parts of the Moine Thrust in the Scottish Highlands ; Alfred Elis Törnebohm in the Scandinavian Caledonides and R. G. McConnell in the Canadian Rockies. The realisation that older strata could, via faulting, be found above younger strata

1147-502: The sedimentary package, dipping under the wedge while the overlying sediments are often lifted up against it. Backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust. Thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. A thrust fault

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1184-465: The subduction zone and accreted to the continental margin. Since the Late Devonian and Early Carboniferous periods, some 360 million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes, each producing a mountain-building event. The piecemeal addition of these accreted terranes has added an average of 600 km (370 mi) in width along

1221-505: The thrust-belt development in the incipient Taiwan arc-continent collision zone. In accretionary wedges, seismicity activating superimposed thrusts may drive methane and oil upraising from the upper crust. Mechanical models that treat accretionary complexes as critically tapered wedges of sediment demonstrate that pore pressure controls their taper angle by modifying basal and internal shear strength. Results from some studies show that pore pressure in accretionary wedges can be viewed as

1258-563: The underlying block is exposed only in a relatively small area. When erosion removes most of the overlying block, leaving island-like remnants resting on the lower block, the remnants are called klippen (singular klippe ). If the fault plane terminates before it reaches the Earth's surface, it is called a blind thrust fault. Because of the lack of surface evidence, blind thrust faults are difficult to detect until rupture. The destructive 1994 earthquake in Northridge, Los Angeles, California ,

1295-507: The western margin of the North American continent. The topographic expression of the accretionary wedge forms a lip, which may dam basins of accumulated materials that, otherwise, would be transported into the trench from the overriding plate. Accretionary wedges are the home of mélange , intensely deformed packages of rocks that lack coherent internal layering and coherent internal order. The internal structure of an accretionary wedge

1332-439: Was arrived at more or less independently by geologists in all these areas during the 1880s. Geikie in 1884 coined the term thrust-plane to describe this special set of faults. He wrote: By a system of reversed faults, a group of strata is made to cover a great breadth of ground and actually to overlie higher members of the same series. The most extraordinary dislocations, however, are those to which for distinction we have given

1369-456: Was caused by a previously undiscovered blind thrust fault. Because of their low dip , thrusts are also difficult to appreciate in mapping, where lithological offsets are generally subtle and stratigraphic repetition is difficult to detect, especially in peneplain areas. Thrust faults, particularly those involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts mainly propagate along zones of weakness within

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